Turbochargers extract energy from a vehicle exhaust to drive a compressor to deliver air at high density to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower. Tighter regulation of engine exhaust emissions has led to an interest in boost devices capable of delivering ever higher pressure ratios. One way to achieve this is to drive the compressor wheel at higher tip speeds, typically translating to 80,000 RPM to 300,000 RPM, depending upon the diameter of the compressor wheel. Not only high rotational speeds, but also shaft forces to rapidly accelerate the compressor wheel, create high tensile loading of the compressor wheel. This loading is especially severe particularly near the bore. It is conventional to reinforce the backwall of a compressor wheel with a central bulge.
Compared to a compressor wheel, a turbine wheel is usually made of a higher value alloy, able to withstand the high temperatures and corrosive gasses to which the turbine wheel is exposed. The turbine wheel also differs from a compressor wheel in the manner of joining to the shaft, i.e., while a compressor wheel typically has a through-going bore by which it is seated on a shaft, and is fixed to the shaft via a nut, a turbine wheel is solid and is materially fixed to the shaft, e.g., by welding or brazing. Turbine wheel backwalls also differ from compressor wheel backwalls. Turbine wheels backwalls are conventionally substantially flat. See US Patent Application 2010/0003132 (Holzschuh) assigned to the assignee of the present application, which forms the basis for FIGS. 1 and 2.
Since the turbine wheel and compressor wheel are fixed to the same shaft, the turbine wheel must spin at the same high RPM as the compressor wheel. The turbine wheel is also subjected to repetitive stresses and can experience low cycle fatigue failure. There is thus a need to further guard against the possibility of low cycle fatigue failure in turbine wheels.
The commercial turbocharger industry is cost driven. While there is a need to reduce to low cycle fatigue failure, this objective must be accomplished economically, i.e., without resort to high cost measures such as multiple-alloy wheel manufacturing techniques, exotic alloys, five-axis milling from billet, time-consuming cold-working to remove surface defects, etc.
It has recently been discovered that compressor wheels provided with a slightly longer, profiled hub end have improved life against low cycle fatigue. Compressor wheels with this design have been referred to as “superback”. To accommodate the added length of the superback compressor wheel, the industry found the need to redesign other associated features of the turbocharger such as flinger and diffuser.
Although there are significant structural, metallurgical, and joining differences between compressor wheels and turbine wheels, the present inventors investigated wither increased hub length could also provide benefits to turbine wheels with regard to prevention of low cycle fatigue. Since turbine housings are generally designed to receive turbine wheels with flat backs, and since it is conventional practice to balance turbine wheels by removing material from a flat region of the backwall, there was a question as to how to design a “superback” turbine wheel to, on the one hand, possibly provide the desired benefit, and on the other hand, cause the minimum disruption to the industry, e.g., allow the industry to continue with conventional balancing processes, and to incorporated into the available line of turbine housing with minimum re-design and re-engineering of cooperating turbocharger components.
An initial turbine wheel superback design provided a generally conical transition between an elongated weld hub and the flat backwall of the turbine wheel (FIG. 2). This turbine wheel superback design was tested commercially and was found to meet expectations. The present inventors nevertheless investigated to see whether even greater improvements could be achieved. The inventors considered different alloys, mechanical surface treatments, chemical surface treatments, coatings, heat treatments and other options.